The present invention relates to an optical device for tracking lateral movement of an object. In particular, the present invention relates to an optical tracking device to track the eye movement during a laser surgery on the cornea.
In a cornea surgery with a small laser beam, such as photo-refractive keratectomy or laser assisted in-situ keratomileuses, a fast and accurate eye tracking device is usually required to track the position of a patient""s eye under surgery. With such a tracking device, the surgical laser beam can be delivered to predetermined positions on the eye even when the eye moves during the surgery. An example of this tracking device is described in U.S. Pat. No. 5,632,742 to Frey, et al. Another example is presented in U.S. Pat. No. 5,620,436 to Lang and Clouts.
These devices are optical systems operated in the infrared spectral range. To achieve a reliable eye tracking with high accuracy and high speed, it is essential to obtain on the cornea a well-defined reference of which the position can be determined precisely and quickly by an optical imaging system. Natural structures of the eye, such as the pupil and the limbus, do not always provide a reliable reference for this purpose. For instance, the centroid of the pupil moves when the pupil changes its size. In addition, these natural structures may be disturbed during the surgery.
High contrast masks have been proposed to apply onto the cornea to provide reliable references for a variety of eye-tracking devices. Frey et al. disclosed in U.S. Pat. No. 5,632,742 an ink ring affixed on the patient""s eye. Lang and Clouts described in U.S. Pat. No. 5,620,436 a ring-shape aiming-fixture applied onto the patient""s eye.
The main advantage of a ring shape mask is that it can be easily placed around the center of the cornea, where the laser surgery is taken place. On the other hand a ring shape mask is not convenient in many situations. One example is for laser assisted in-situ keratomileuses. In this type of surgery, a mechanical device called automated microkeratone laminates a thin layer of the cornea from the central part of the patient""s eye. This layer is attached to the cornea by an uncut hinge and is flapped over to allow laser ablation on the corneal bed. This flap makes it difficult to apply a ring shape mask on the eye.
The present invention contemplates a small reference disk with a retro-reflective surface to enable fast and reliable tracking of lateral movement of an object such as an eye. The reference disk is affixed onto an object to be tracked. The disk has a retro-reflective surface to enhance significantly the backward scattering of an incident beam. An illumination beam illuminates the reference disk from a direction. An imaging optics collects the backward scattering to form a bright image spot of the reference disk. The lateral position of such a bright image spot can be detected by a single-element positioning-detector. An electronic circuit coupled to the positioning detector can then generate positioning signals of the reference disk and thus enable the tracking of the lateral movement of the object.
In the embodiments presented in this application, a retro-reflective disk and a single-element positioning-detector are implemented to provide fast eye tracking for refractive laser surgery on the cornea. In these embodiments, a retro-reflective disk of a few millimeters in diameter is affixed on the cornea near and outside the surgery area. An infrared light source located near the visual axis illuminates the eye and the reference disk from a working distance of about 25 cm. An imaging optics forms an image of the cornea area on a single-element positioning-detector. The strong backward scattering from the retro-reflective disk produces a bright spot in a basically dark background image. When the eye moves, the reference disk moves with the eye and the bright image spot moves on the positioning detector. An electronic circuit that reads in the output from the positioning detector generates positioning signals of the bright image spot and, thus, provides information on the eye movement. A control circuit can then use these positioning signals to control a beam steering mechanism to direct the surgical laser beam to follow the movement of the eye.
The reference disk may include a substrate and a retro-reflective surface according to one embodiment. The second surface of the disk is attachable to the cornea without slipping. The substrate can be made of paper or other materials, which are harmless to the cornea and durable for sterilization. The reference disk is preferably a disposable item.
The application of a retro-reflective reference disk simplifies the positioning detection of the eye to the positioning detection of a bright image spot. Consequently, single-element positioning-detector and simple electronics can be used to achieve fast and sensitive tracking of the eye movement. A prototype shows that positioning detection with a retro-reflective disk and a single-element positioning-detector can be faster than 10 kHz and sensitive to a few microns. In comparison, a CCD camera based positioning detection requires expensive frame grabber and sophisticate data-processing electronics. The up-date rate of a CCD camera is typically limited to 30 to 60 Hz. These and other aspects and advantages of the present invention will become more apparent in the following drawings, detailed description, and claims.